It is reported that the Messenger of Allah, peace and blessings of Allah be upon him, said:
“Fear Allah with regard to prayer and those you own  .” This hadeeth was transmitted by al-Khatib al-Bagadadi in Tariq al Baghdad (10/169) from the words of Umm Salama, al-Tahawi in “Mushkil al-Athar” (4/235-236).
اتقوا الله في الصلاة وما ملكت أيمانكم
(صحيح) … [خط] عن أم سلمة. الصحيحة 868)
Similar Hadith in Musnad Ahmad 585, Al- Adab Al-Mufrad 158
Sheikh al-Albani called the hadith authentic. See “Sahih al-Jami ‘al-saghir” 105. See “al-Silsila as Sahiha 868”
These were the last words of Messenger of Allah (ﷺ)
 This refers to slaves.