Musnad Ahmad 28:18

Sufyan narrated to us, from ‘Amr, from Abu Qabus, from ‘Abdullah ibn ‘Amr ibn al-‘As, attributing it to the Prophet, may God bless him and grant him peace, who said: "The merciful ones are shown mercy by the Most Merciful. Show mercy to those on earth, and the inhabitants of the heavens will show mercy to you. And the womb (al-rahim) is a branch (shujnah) from the Most Merciful; whoever maintains it, He maintains him, and whoever severs it, He severs him."