Mozart esteemed Handel and in 1789 he was commissioned by Baron Gottfried van Swieten to rearrange Messiah (HWV 56).
This work likely influenced the composition of Mozart's Requiem; the Kyrie is based on the "And with His stripes we are healed" chorus from Handel's Messiah, since the subject of the fugato is the same with only slight variations by adding ornaments on melismata.
However, the same four-note theme is also found in the finale of Haydn's String Quartet in F minor (Op.
20 No.
5) and in the first measure of the A minor fugue from Bach's Well-Tempered Clavier Book 2 (BWV 889b) as part of the subject of Bach's fugue, and it is thought that Mozart transcribed some of the fugues of the Well-Tempered Clavier for string ensemble (K. 404a Nos.
1-3 and K. 405 Nos.
1-5), but the attribution of these transcriptions to Mozart is not certain.

Some believe that the Introitus was inspired by Handel's Funeral Anthem for Queen Caroline, HWV 264.
Another influence was Michael Haydn's Requiem in C minor which he and his father were viola and violin players respectively at the first three performances in January 1772.
Some have noted that Michael Haydn's Introitus sounds rather similar to Mozart's, and the theme for Mozart's "Quam olim Abrahae" fugue is a direct quote of the theme from Haydn's Offertorium and Versus.
In Introitus m. 21, the soprano sings "Te decet hymnus Deus in Zion".
It is quoting the Lutheran hymn Meine Seele erhebet den Herren.
The melody is used by many composers e.g. in Bach's cantata Meine Seel erhebt den Herren, BWV 10 but also in Michael Haydn's Requiem.

Felicia Hemans' poem "Mozart's Requiem" was first published in The New Monthly Magazine in 1828.
Mozart esteemed Handel and in 1789 he was commissioned by Baron Gottfried van Swieten to rearrange Messiah (HWV 56). This work likely influenced the composition of Mozart's Requiem; the Kyrie is based on the "And with His stripes we are healed" chorus from Handel's Messiah, since the subject of the fugato is the same with only slight variations by adding ornaments on melismata. However, the same four-note theme is also found in the finale of Haydn's String Quartet in F minor (Op. 20 No. 5) and in the first measure of the A minor fugue from Bach's Well-Tempered Clavier Book 2 (BWV 889b) as part of the subject of Bach's fugue, and it is thought that Mozart transcribed some of the fugues of the Well-Tempered Clavier for string ensemble (K. 404a Nos. 1-3 and K. 405 Nos. 1-5), but the attribution of these transcriptions to Mozart is not certain.
Some believe that the Introitus was inspired by Handel's Funeral Anthem for Queen Caroline, HWV 264. Another influence was Michael Haydn's Requiem in C minor which he and his father were viola and violin players respectively at the first three performances in January 1772. Some have noted that Michael Haydn's Introitus sounds rather similar to Mozart's, and the theme for Mozart's "Quam olim Abrahae" fugue is a direct quote of the theme from Haydn's Offertorium and Versus. In Introitus m. 21, the soprano sings "Te decet hymnus Deus in Zion". It is quoting the Lutheran hymn Meine Seele erhebet den Herren. The melody is used by many composers e.g. in Bach's cantata Meine Seel erhebt den Herren, BWV 10 but also in Michael Haydn's Requiem.
Felicia Hemans' poem "Mozart's Requiem" was first published in The New Monthly Magazine in 1828.
Requiem (Mozart)