Authentic Supplication Number 166.

The Prophet  said: “If one of you loves his brother then let him inform him of that.”

Reporters.
‘Al-`Adawee said it was sound, and that it was reported by ‘Ibn ‘As-Sunni in ‘Al-Yawm wal-Laylah, #196. His #197 serves as testimonial for this hadeeth.