We have no historical evidence for that.

Wallace's first recorded speech comes from two Dominican friars sent to treat with him at the Battle of Stirling in 1297: "Tell your commander that we are not here to make peace but to do battle, defend ourselves and liberate our kingdom. Let them come on, and we shall prove this in their very beards."

Beyond that, Wallace's recorded words get pretty boring. The Hastings Letter (a letter from Robert Hastangis, or Hastings, to Edward I, c. 1300) paraphrases Wallace talking about land rights, and the Lübeck Letter, composed by William Wallace and Andrew Moray, discusses safe passage for merchants.

Our primary sources for Wallace's actions come from chronicles (for example, the Rishanger, Lanercost, and Guisborough Chronicles; John of Fordun's account of the Battle of Falkirk; Thomas Grey's Scalacronica). Chronicles are valuable historical resources but are not known for their verbosity. They mostly just contain short entries of bare-bones statements about events, and usually don't include much if any dialogue. Here's a good example, from the Scalacronica:

'[...] in the month of May William Wallace was chosen by the commons of Scotland as leader to raise war against the English, and he at the outset slew William de Heselrig at Lanark, the King's Sheriff of Clydesdale. The said William Wallace came by night upon the said sheriff and surprised him, when Thomas de Gray, who was at that time in the suite of the said sheriff, was left stripped for dead in the mellay when the English were defending themselves. The said Thomas lay all night naked between two burning houses which the Scots had set on fire, whereof the heat kept life in him, until he was recognised at daybreak and carried off by William de Lundy, who caused him to be restored to health. And the following winter, the said William Wallace burnt all Northumberland.'

Burning 'all of Northumberland' only warrants a short sentence! Chroniclers on the whole didn't devote much time or parchment to speeches. There are a couple of rolls that list Wallace's execution sentence and an 18th-century copy of a medieval Latin account of his trial (sadly the original manuscript was burnt), but that line doesn't appear in any of them.