And being assembled together with them, commanded them that they should not depart from Jerusalem, but wait for the promise of the Father, which, saith he, ye have heard of me. For John truly baptized with water; but ye shall be baptized with the Holy Ghost not many days hence. When they therefore were come together, they asked of him, saying, Lord, wilt thou at this time restore again the kingdom to Israel? And he said unto them, It is not for you to know the times or the seasons, which the Father hath put in his own power. But ye shall receive power, after that the Holy Ghost is come upon you: and ye shall be witnesses unto me both in Jerusalem, and in all Judaea, and in Samaria, and unto the uttermost part of the earth. And when he had spoken these things, while they beheld, he was taken up; and a cloud received him out of their sight. And while they looked stedfastly toward heaven as he went up, behold, two men stood by them in white apparel; Which also said, Ye men of Galilee, why stand ye gazing up into heaven? this same Jesus, which is taken up from you into heaven, shall so come in like manner as ye have seen him go into heaven. Then returned they unto Jerusalem from the mount called Olivet, which is from Jerusalem a sabbath day's journey. And when they were come in, they went up into an upper room, where abode both Peter, and James, and John, and Andrew, Philip, and Thomas, Bartholomew, and Matthew, James the son of Alphaeus, and Simon Zelotes, and Judas the brother of James. These all continued with one accord in prayer and supplication, with the women, and Mary the mother of Jesus, and with his brethren.

(Acts 1:4-15)

Troparion

Thou hast ascended in glory, O Christ our God, having made joyful Thy disciples
by the promise of the Holy Spirit, which was announced unto them in the blessing;
for Thou art the Son of God, the Redeemer of the world.

Epistle on the Ascension of the Lord by His Eminence Metropolitan Laurus, First Hierarch of the Russian Orthodox Church Outside of Russia (2007)

The Ascension of Our Lord (Fr.V.Potapov, 2004)

Lessons from the Fathers