I mean those slavic tribes that became part of Rus, from their earliest history.
In short answer, definitely since pre-Christianization period.
A few Arabic sources since the early 10th century, including famous Ibn Fadlan, mention the slave trades mainly conducted by the Rus' in now Russian waterways like Dnieper and Volga latest since around 800 CE (at least up to the middle of the 10th century). The inflow of Islamic silver originated either from Abbasid Empire in Baghdad or from the Samanids in Central Asia presupposed the counter-waves of the export of slaves and furs from North-Western Eurasia, especially around the Baltic.
A recent study estimates the amount of excavated Islamic silver coins, dirhams, as 400,000 coins in total during ca. 670- 1090 - 87,000 have been found in Sweden, 37,000 from now Poland, and 207,000 from Russia, Ukraine, and Belarus. This roughly corresponds with the price for ca. 1,000 slaves per a year, only for the hitherto excavated coins only! Much, much more silver coins must have been lost somewhere in north-western Eurasia in course of time (Hansen 2020: 90).
The following are some examples of these texts (Lunde & Stone ed. & trans. 2010: 122f., 126, 47):
- 'The Magyars are pagans, worshipping fire. They make piratical raids on the Slavs and follow the coast of the Black Sea with their captives to a port in Byzantine territory named Karkh (Kerch).......When the Magyars bring their prisoners to Karkh, the Greeks go there to trade. The Magyars sell their slaves and buy Byzantine brocade, woollen rugs and other products of the Byzantine Empire' (Ibn Rusta on the Magyars).
- 'The Rus' (Rusiya) raid the Saqaliba [the Europeans, including the Slavs], sailing in their ship until they come upon them. They take them captive and sell them in Khararan and Bulkar (Bulghar).....' (Ibn Rusta on the Rus').
- 'When they [Rus'] arrive from their land, they anchor their boat on the Itil, which is a great river, and they build large wooden houses on the banks......With them, there are beautiful slave girls, for sale to the merchants' (Ibn Fadlan on the Rus').
These texts are at least suffice to say that the slavery and the slave trade was well-known in now Russia around 900. One might argue that these Rus' mentioned in the cited sources were primarily the Scandinavians, but as I explained before in How come there isn't a greater Nordic/Viking influence on Russia and its language even though the Kievan Rus' was at first ruled by Varangians?, the recent academic consensus emphasizes the Scandinavian-Slavic-Christian-steppes cultural hybridity of the ruling elites of arising Kievan Rus' in the turn of the first millennium.
>In what form?
Slave taking (hunt) or legal definition of slaves?
If the latter is OP is asking for answer, the oldest extant evidence, the Pravda Rus'skaia (linked to the English translation by Daniel Kaiser), unfortunately dates only back to the middle of the 11th century, after Christianization of the Rus'.
>did slavic tribes before and after emergence of Rus ever capture foreign slaves?
This can be a bit tricky part.
As I wrote before in Where did the Piast dynasty originate from?, the Poles of the later Piast dynasty probably sold their vanquished neighboring group of the Slavs to accumulate power and wealth mainly from the 930s and 950s, and their activity reflected in the excavated silver coins found in now Greater Poland region. This was primarily 'the Slavs sold neighboring vanquished Slavs', however.
On the other hand, Korpela counts ca. 50 medieval Russian raids into the land of the still pagan Finnish peoples (now in southern Lapland, central-southwestern Finland, and Karelia as well) since the middle of the 11th century found mainly in Novgorodian chronicles (Korpela 2019: 50). He also mentions a few examples of raiding by the Novgorodians, and their taking the Turkic nomadic peoples as war hostages since the early 12th century, as following (Korpela 2019: 51):
- 'Svyatopolk, Volodimir and David and the whole Russian Land to a man went against the Polovets people and defeated them and took their children, and rebuilt the fortified towns of Surtov and Sharukan (1111 CE =A.M. 6619, in: Beazley & Shakhmatov trans. 1914: 8).
- 'Vsevolod with the men of Novgorod went against the Chud people in the winter during the Feast; them he slaughtered, their dwellings he burned, and their wives and children he brought home' (1130 CE=A.M. 6638, in: Ibid., 12).
So, I suppose that to hunt slaves as well as to secure the tribute from pagan hunter-gathering Finnish peoples at least constituted one motive to keep on raiding into the land of neighboring peoples for the Rus' after their Christianization.
[Added]: Concerning the persistence of raids for slaves and slave trade in later medieval and early modern Russia, the following blog entry by /u/mikedash is really worth checking: https://mikedashhistory.com/2015/01/15/blonde-cargoes-finnish-children-in-the-slave-markets-of-medieval-crimea/
(Additional) References, in addition to the cited ones in the linked posts:
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[Edited (2nd times)]: Corrects some citing style mistakes, and the published year of Hansen 2020 (sorry again).