Abu Bakr’s son Abdullah was hit by an arrow during the Battle of Ta’if after the conquest of Mecca. This arrow did not kill him but wounded him severely, and it continued to cause problems until he eventually died from it in Madinah over a year later, 40 days after the death of the Prophet ﷺ.
Later, when Abu Bakr was the Caliph, the tribe of Ta’if, Banu Thaqifah, came to visit him. Abu Bakr had kept the arrow, so he took it out and asked the tribe of Thaqif: “Do you recognise who this arrow belongs to?”
So they passed it around until one of them, Sa’id ibn Ubayd, said “This is my arrow; I recognise it. I’m the one who carved it and sharpened it and feathered it and shot it.” In those days each arrow was custom made and hand made, and they would be collected and used again. They did not realise the significance of the arrow and who had been affected by it!
So Abu Bakr says, and look at his amazing iman:
“This is the arrow that killed my son Abdullah. So alhamdulillah to the One who honoured him through you, and did not humiliate you through him, for that is better for the both of you.”
This is because his son was honoured with martyrdom through Sa’id, and Sa’id would be forgiven for that because he became Muslim afterwards, so his sins before are forgiven. This is the amazing iman of Abu Bakr al-Siddiq.
Source: Yasir Qadhi, Lives of the Sahaba, Abu Bakr, Part 3