Fiqh-us-Sunnah Volume 4, Funerals and Dhikr, Fiqh 4.038.

The prerequisites for a funeral prayer are the same as for the obligatory prayers. Anyone intending to offer funeral prayer must be in a state of purity, be free from all minor and major impurities, must cover his or her “awrah,” and stand facing the direction of the Ka’bah.

Malik reported from Nafi’ that Abdullah Ibn ‘Umar used to say: “One should not offer a funeral prayer unless he is in a state of purity.”

The funeral prayer differs from the prescribed prayers in that there is no fixed time for offering it. It may be offered at any time, including the times when regular prayers may not be offered. This is the opinion of the Hanafi and Shafi’i schools. Ahmad, Ibn Al-Mubarak, and Ishaq dislike offering a funeral prayer at sunrise, at noon when the sun is at its zenith, and at dusk when the sun is about to set, except in cases when it is feared that if delayed, the body might decompose.