Sederunt principes, et adversum me loquebantur: et iniqui persecuti sunt me: adjuva me, Domine Deus meus, quia servus tuus exercebatur in tuis justificationibus. * Beati immaculati in via, qui ambulant in lege Domini.Princes sat, and spoke against me: and the wicked persecuted me: help me, O Lord my God, for Thy servant was employed in Thy justifications.* Blessed are the undefiled in the way, who walk in the law of the Lord. (Psalm 118:23,86, 23, 1 from the Introit of Mass)
Omnipotens sempiterne Deus, qui primitias Martyrum in beati Levitae Stephani sanguine dedicasti: tribue, quaesumus; ut pro nobis intecessor exsistat, qui pro suis etiam persecutoribus exoravit Dominum nostrum Jesum Christum Filium tuum.
O almighty and everlasting God, who didst consecrate the first-fruits of the martyrs in the blood of the blessed Levite Stephen: grant, we beseech Thee, that he may intercede for us, who entreated even for his persecutors, our Lord Jesus Christ Thy Son.
From a Sermon of the Bishop Saint Augustine.Christ, the Chief of Martyrs, also suffered for us, leaving us an example, that ye should follow his steps. And the steps of the Passion of this same Chief of Martyrs did blessed Stephen follow close, in that he was stoned by the Jews for his confession of Christ, and thereby gained the crown which pertained to him in virtue of his name. For in the Greek language the word Stephen doth signify a crown. Thus already bore the name Crown, whereby was foretold the palm of martyrdom; which same he was to gain when he was stoned to death. Yet notwithstanding, he sought not to draw down vengeance on his persecutors, but contrariwise prayed God to forgive them.
(Sermo 2 de S. Stephano)
The Catholic Encyclopaedia on St Stephen: http://www.newadvent.org/cathen/14286b.htm
Reference for the sermon from St Augustine: http://www.breviary.net/propseason/christmas/propseasonchri0102.htm#Matins