Bukhari

Hadith Commentary:
The meaning of "witr" is single, alone, odd; its opposite is "shaf‘" which means even or paired.
The Messenger of Allah (sallallahu alayhi wa sallam) sometimes performed witr as seven rak‘ahs, sometimes five, sometimes three, and sometimes as a single rak‘ah.
Abu Ayyub (radi Allahu anhu) narrates that the Messenger of Allah (sallallahu alayhi wa sallam) said:
"Al-witru haqq ‘ala kulli muslim, man ahabba an yutira bikhamsin falyaf‘al, wa man ahabba an yutira bithalathin falyaf‘al, wa man ahabba an yutira biwahidatin falyaf‘al." (Narrated by Abu Dawud, al-Nasa’i, and Ibn Majah)
That is, the witr prayer is a right and established upon every Muslim; so whoever wishes may perform witr as seven rak‘ahs, whoever wishes may perform five rak‘ahs, whoever wishes may perform three rak‘ahs, and whoever wishes may perform one rak‘ah.
According to the narration of Ibn ‘Umar (radi Allahu anhuma), the Prophet (sallallahu alayhi wa sallam) said: "Al-witru rak‘atun min akhir al-layl." (Narrated by Muslim)
That is, the witr prayer is in the last part of the night and is one rak‘ah. When the Prophet (sallallahu alayhi wa sallam) performed witr as five rak‘ahs, he would not sit (for tashahhud) in between, but only in the last rak‘ah would he sit (Narrated by Muslim).
Thus, performing one rak‘ah of witr is permissible, correct, and in fact the Sunnah of the Prophet (sallallahu alayhi wa sallam). Those who object to people performing one rak‘ah of witr are themselves mistaken; likewise, it can be performed as three, five, or up to seven rak‘ahs.
The correspondence between the hadith and the chapter is that the Messenger of Allah (sallallahu alayhi wa sallam) placed his hand of mercy and supplication on the head of ‘Abdullah bin Thalabah (radi Allahu anhu) out of compassion and prayer.

Hadith Commentary:
(1)
All these hadiths express the reality that the Messenger of Allah (sallallahu alayhi wa sallam) showed special compassion towards children. He would affectionately pass his hand over their heads and supplicate for blessings for them. There were also some children with whom, in a playful manner, the Messenger of Allah (sallallahu alayhi wa sallam) would sprinkle the water from his mouth onto their faces. The effects of the supplications of the Messenger of Allah (sallallahu alayhi wa sallam) were visibly apparent.

(2)
If any nursing child urinated on his clothes, he would not take offense; rather, he would call for water and himself pour it over the soiled garment. The son of Abdullah bin Salam (radi Allahu anhu) narrates that the Messenger of Allah (sallallahu alayhi wa sallam) named me Yusuf, seated me in his lap, and affectionately passed his hand over my head. (Musnad Ahmad: 6/6)