Solat Tahajjud

Iyas ibne Mua'wiyah Al Muzani Rahimahullah narrates that Rasulullah SAW said: Be sure to offer solat Tahajjud though it may be as short as a goats milking. Any solat offered after 'Isya will indeed be counted as Tahajjud.
(Tabarani, Majma'uz Zawaid)

Uqbah ibne 'Amir RA narrates: I heard Rasulullah SAW saying One out of two persons of mu Ummah, gets up at night and motivates himself for wudhu despite feeling reluctant to do so, since Satan has tied knots on him. When he washes his two hands in wudhu, one knot is loosened, and when he washes his face, another knot is loosened, and when he wipes his head with wet hands, another knot is loosened. Allah Azza Wajala says to those behind the veil - His angels: Look at My slave! How much hardship he is enduring in dealing with his Nafs (inner desires) so now whatever this slave asks of Me, it will be granted to him.
(Musnad Ahmad, Fath ul Rabbani