من لا يحضره الفقيه, volume 3, page 37

Hadith.3279 - It is narrated from Sabbah Al-Muzani, who raised it (in chain of narration): Two men came to Commander of the Faithful (as). One of them said: "O Commander of the Faithful, this man and I went out together. I brought three loaves of bread, and he brought five loaves of bread. We had lunch together, and a man passed by us, so we invited him to join us for lunch. He came and ate with us. When we finished, he gifted us eight dirhams and left. I said to this man: 'Divide the dirhams with me.' He said: 'I will not do so except based on the proportion of the shares of the bread.' Commander of the Faithful (as) said: 'Go and reconcile between yourselves.' The man replied: 'O Commander of the Faithful, he refuses to give me more than three dirhams while he takes five dirhams. Judge between us.' Commander of the Faithful (as) said to him: 'O servant of Allah (swt), do you know that three loaves of bread equal nine thirds?' He said: 'Yes.' Imam (as) continued: 'And do you know that five loaves of bread equal fifteen thirds?' He said: 'Yes.' Imam (as) said: 'You ate from your nine-thirds eight-thirds, leaving you one-third. And this man ate from his fifteen-thirds eight-thirds, leaving him seven-thirds. The guest ate seven-thirds from this man's bread and one-third from your bread. Thus, each of you consumed eight-thirds. This man is entitled to seven dirhams, one dirham for each remaining third of bread he contributed, and you are entitled to one dirham for the one-third you contributed. So, take one dirham, and give this man seven dirhams.'"

Translation: Bab Ul Qaim Publications·Thaqalayn link·The Arabic above is the original.