Musannaf Ibn Abi Shayba 33:1224

'Abdullah bin Numayr narrated to us, he said: Malik bin Mighwal narrated to us, from Talha, from Mujahid, who said: "When a man meets a man and smiles in his face, their sins fall away from them just as the wind scatters dry leaves from a tree." A man said: "Woe to you! This is an easy deed." He said: "Have you not heard His saying, the Exalted: 'If you had spent all that is in the earth, you could not have brought their hearts together'?"