On the authority of Aboo Hurayrah (may Allaah be pleased with him), the Prophet (sallallaahu 'alayhe wa sallam) used to congratulate newlyweds, saying:

(( بَارَكَ اللهُ لَكَ، وَبَارَكَ عَلَيْكَ،
وَجَمَعَ بَيْنَكُمَا فِي الْخَيْرِ. ))

"May Allaah bless you,
send down His Blessings upon you,
and unite you both upon goodness."

It was collected by Aboo Daawood (2130), at-Tirmithee (1091), and Ibn Maajah (1905).  This is at-Tirmithee's wording, who called it hasan-saheeh.  Al-Albaanee graded it saheeh.