Sayyiduna Anas bin Malik (may Allah be pleased with him) said: When Sayyidah Zaynab bint Rasulullah (peace and blessings be upon him) passed away, we went out with him (peace and blessings be upon him), and we saw that the Messenger of Allah (peace and blessings be upon him) was deeply grieved and troubled, so we said: We will not speak to him (peace and blessings be upon him). When we reached the grave, we found that the digging of its niche (lahd) had not yet been finished. The Messenger of Allah (peace and blessings be upon him) sat down and we sat around him (peace and blessings be upon him). He (peace and blessings be upon him) spoke to himself for a while and kept looking towards the sky. Then the grave was ready, and the Messenger of Allah (peace and blessings be upon him) went down into it, and I saw that his (peace and blessings be upon him) grief was increasing. Then, when he (peace and blessings be upon him) had finished and came out, I saw that his (peace and blessings be upon him) distress had lifted and he (peace and blessings be upon him) was smiling. We said: O Messenger of Allah, we saw you grieved and troubled, and we could not even speak to you; then we saw that your distress had lifted. What was the reason for that? He (peace and blessings be upon him) said: "I was thinking of the narrowness of the grave and its distress, and of Zaynab's weakness, and that weighed heavily on me, so I supplicated to Allah to lighten it for her, and He did so. And indeed she was squeezed with a squeeze that everything between the east and the west heard, except the jinn and mankind."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19257