Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.089.

Whoever catches a rak’ah of prayer before its time has expired has caught the entire prayer in its time. Abu Hurairah reported that the Prophet, upon whom be peace, said, “Whoever catches one rak’ah of the prayer has caught the prayer.” (Related by “the group.”) This refers to any of the prayers. Al-Bukhari has recorded, “Whoever of you catches one prostration of the afternoon prayer before the sun has set should complete his prayer. If one of you catches one prostration of the morning prayer before the sun has risen, he should complete his prayer.” Here the meaning of prostration is rak’ah. The clear meaning of the hadith is that one who catches one rak’ah of the morning or afternoon prayer should complete the prayer even if the sun is setting or rising, also those are times in which it is not liked to pray. If one rak’ah is performed, then the prayer is to be completed and the obligation of prayer will have been fulfilled, although it is not allowed to intentionally delay those prayers until such times.