Leo the Great

Blessed tears, O holy Apostle, which had the virtue of holy Baptism in washing off the sin of thy denial. The right hand of the Lord Jesus Christ was with thee to hold thee up before thou wast quite thrown down, and in the midst of thy perilous fall, thou receivedst strength to stand. The Rock quickly returned to its stability, recovering so great fortitude, that he who in Christ's passion had quailed, should endure his own subsequent suffering with fearlessness and constancy.

Catena Aurea by Aquinas ↗

Thomas Aquinas

And immediately the cock crew. Here the repentance of Peter is treated. And first, what moved him or excited him to repentance is set forth; secondly, his repentance, at and going forth he wept bitterly. Two things are touched upon by which he was stirred. First, the crowing of the cock; hence and immediately the cock crew. …

Commentary on Matthew ↗