It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) led the funeral prayer over those slain at Uhud. At first he said nine takbirs over each, then seven each, then four each, until he joined Allah, the Mighty and Majestic.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11355