Midrash su Salmi 119:106

R. Gidel in the name of Rab said (Fol. 8) "Whence do we infer that one may bind himself with an oath to perform a commandment? It is said (Ps. 119, 106) I have sworn, and I will perform, etc." Behold every one is bound by the Sinaic oath to perform the commandments? We must therefore say that he means it is proper to swear [to perform a religious act] in order to urge upon himself its fulfillment. R. Gidel in the name of Rab said further: "Whoever says to his associate, let us get up in the morning and study this chapter, the one that proposed it is supposed to get up first, as it is said (Ezek. 3, 22) And He said unto him, arise, go forth into the valley, etc. And I arose, and went forth into the valley; and, behold, there was the glory of the Lord standing."