Which son did Abraham sacrifice?

None. When Isaac was in his twenties, Abraham followed God's command and prepared to sacrifice him on an alter, but God stopped him before it happened.(Genesis 22:9-13). This account was included to show Abraham's faithfulness and trust in God, making him a worthy patriarch for God's original 'chosen people', the Nation of Israel(Deuteronomy 10:12-14 + 14:2)(Isaiah 41:8; James 2:23). It also pictured the depth of love God showed when allowing HIS son to be sacrificed in behalf of sinful mankind(Hebrews 11:17-19)(James 2:21). Along with that, it showed that if we are obedient to God in all things, God will not forget the 'love we show for his name'(1 John 5:3)(1 John 4:10)(Hebrews 6:10).