The prayer of the Lord is mercy, and the prayer of the angels is seeking forgiveness. (1)

492 - Abu Dawud Sulayman ibn Salm al-Balkhi narrated to us, saying: al-Nadr ibn Shumayl informed us, from Abu Qurrah al-Asadi, from Sa'id ibn al-Musayyib, from 'Umar ibn al-Khattab, who said: Indeed, supplication remains suspended between the heaven and the earth, and nothing of it ascends until you send blessings upon your Prophet, may Allah bless him and grant him peace. (2)

Al-'Ala' ibn 'Abd al-Rahman: he is Ibn Ya'qub, the freed slave of al-Huraqah. Al-'Ala' is among the Followers; he heard from Anas ibn Malik and others.

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(1) See Fadl al-Salat 'ala al-Nabi, pp. 80-81, by Isma'il al-Qadi.
(2) Its chain of transmission is weak; Abu Qurrah al-Asadi is unknown.
It was also recorded by Ishaq ibn Rahawayh in his Musnad, as found in al-Busiri's Ittihaf al-Khayrah (8297), from al-Nadr ibn Shumayl, through him.
It was also recorded by Isma'il al-Qadi in al-Salat 'ala al-Nabi (74) via the route of 'Amr ibn Musafir, from a shaykh of the people of 'Amr, from Sa'id ibn al-Musayyib, who said: No supplication is made without sending blessings upon the Prophet, may Allah bless him and grant him peace, before it, except that it remains suspended between the water and the earth. Its chain of transmission is weak; 'Amr ibn Musafir — also said to be 'Umar ibn Musawir — is weak, and his shaykh is unidentified.
On this topic, there is a hadith from Mu'adh ibn Jabal, raised to the Prophet, recorded by Ibn Hibban in al-Majruhin 1/113, and its chain of transmission is weak.
And from 'Ali, raised to the Prophet, recorded by al-Bayhaqi in Shu'ab al-Iman (1576), and its chain of transmission is weak; and from him, stopped at 'Ali, recorded by al-Tabarani in al-Awsat (725) and al-Bayhaqi in al-Shu'ab (1575). Al-Hafiz al-'Iraqi deemed its chain of transmission good in his commentary on al-Tirmidhi, 1/folio 345. Such a statement is not made on the basis of personal opinion nor attained through independent reasoning, as Imam Ibn al-'Arabi said in 'Aridat al-Ahwadhi 2/273.
See also what will come under number (3784).