All Praise to Allah Subhanahu wa Taala and blessings of Him be on Prophet Muhammad peace be upon him His family and companions,
As mentioned in the comment above you could find it here The similar narration is stated in Tirmidhi sharif and regarded as Sahih Hadith.
Sahl bin Sa'd narrated that the Messenger of Allah (s.a.w) said:
"Whoever guarantees for me what is between his jaws and what is
between his legs, I shall guarantee Paradise for him."(Tirmidhi)
So It means to guard something which is in between jaws i.e Tongue and to guard which is in between legs i.e Private parts.By improper use of these a person could be a sinner,If a person has not control on his tongue that could lead to sins by backbiting,hurting,lying or causing some mischief ,
Sufyan bin 'Abdullah Ath-Thaqafi said: I said: "O Messenger of Allah!
Inform me about a matter that I may hold fast to." He said: 'Say: My
Lord is Allah, then be steadfast.' I said: "O Messenger of Allah! What
do you fear most for me?" So he took hold of his tongue and said:
Similarly Protecting one's private parts means to protect one self from involving adultery is very important,Allah swt has mentioned punishment for such in the Quran.
And those who do not worship any other deity along with Allah, and do
not unjustly kill any living thing which Allah has forbidden, nor
commit adultery; and whoever does this will receive punishment.(Quran
Allah Swt and His Messenger knows best.