Musannaf Ibn Abi Shayba 9:1194

Waki’ narrated to us, from Ubayd Allah ibn Abd al-Rahman ibn Mawhab, from Salih ibn Kaysan, from Umar and Uthman, they both said: "When he is placed [in the grave] while he is in the side of the house in his shrouds, then she has become lawful [for remarriage]."