Musnad Ahmad 1156:102

Zakariya ibn 'Adi narrated to us, he said: 'Ubayd Allah ibn 'Amr informed us, from Zayd ibn Abi Unaysah, from al-Qasim ibn 'Awf al-Shaybani, from 'Ali ibn al-Husayn, he said: Umm Salamah narrated to us, she said: The Messenger of Allah, peace and blessings of Allah be upon him, was in my house, and a man came and said: "O Messenger of Allah, how much is the charity for such and such?" He said: "Such and such." He said: "For indeed, so-and-so has transgressed against me." He said: So they looked into it, and they found that he had transgressed by a *sa'*. The Prophet, peace and blessings of Allah be upon him, said: "So how will it be for you when someone who transgresses against you more severely than this transgression strives?"