Masruq (may Allah have mercy on him) said: We asked Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) about this verse: "And do not think of those who were killed in the way of Allah as dead; rather, they are alive with their Lord, receiving provision" [Surah Al Imran: 169]. He said: "We too asked about it (he said:) The souls of the martyrs are like green birds," then he narrated the rest of the hadith like the narration of al-Thawri.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 8992