It is narrated from Sayyiduna Ka'b bin Malik (may Allah be pleased with him) that a man of the Ansar died, and Umm Mubashshir said: Convey my greetings of peace to my son Mubashshir. Ka'b (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) did not say that; the Messenger of Allah (peace and blessings be upon him) rather said: "The souls of the martyrs are in the form of green birds that hang wherever they wish." She said: Then I seek Allah's forgiveness. He said: This is what I heard.
Hadith Referenceالمعجم الكبير للطبراني / باب الكاف / 16175