Malik bin Hubayrah (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "When those standing in three rows of Muslims pray the funeral prayer over a Muslim and seek forgiveness for him, Paradise becomes binding for him."
So whenever Malik prayed over a funeral and found its attendees few, he would arrange them into three rows and then pray over it. In the narration of Yazid bin Harun: "except that he is forgiven."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنائز / 6905