Wasa'il al-Shi'a, narrating from Ishaq b. 'Ammar: 'I said to Abu Abdullah: 'I enforced upon myself [as a vow] two units of prayer to perform while travelling or at home in thanks to Allah: 'Should I pray them while travelling during the daytime?' He said, 'Yes.' He then said, 'I hate the making of things obligatory; which is when a man makes something obligatory upon himself.' I said, 'I did not make them [the two units] obligatory on myself for Allah, but I made it incumbent for myself, to pray in thanks to Allah, and I did not make it obligatory for myself. So can I leave them, ifl want?' He said, 'Yes.'