Patience for the sake of Allah is an integral part of emaan.
“Oh, you who believe! Be patient and excel in patience and remain steadfast, and be careful of (your duty to) Allah, that you may be successful.” (Aali ‘Imraan, 3:200)
“And those who patiently persevere in seeking the favor of their Lord, and establish Prayer, and spend out of that with which We have provided them, secretly and openly, and repel evil with good; it is these who shall have the best reward of the final abode.” (Ar-Ra’d 13:22)
“And be patient and your patience is only by (the assistance of) Allah.” (An-Nahl, 16:128)
“And for the sake of your Lord, be patient.” (Al-Muddaththir, 74:7)
Patience is of 3 types,
1) Patience in response to a situation that Allah has decreed. For example, loved one dying.
“And bear patiently that which befalls you.” (Luqmaan, 31:17)
2) Patience in restraining yourself from committing sins.
“Peace be upon you, because you patiently persevered. What a joyous destiny.” (13:24)
3) Patience in constantly worshipping Allah – this is the highest form.
“So worship Him and be patient in His service.” (Maryam, 19:65)
“And enjoin prayer on your family and patiently persevere in doing so.” (TaaHaa,20:132)
Patience is manifested in the heart, tongue, and limbs. It is to restrain yourself from things that will cause your harms. It does not mean you must be happy.
When the Prophet(saw)’s son died, the sahabah asked him, ‘Do you cry?’ He replied, ‘Crying is a sign of mercy. We only say that which will please Allah.’
“The eye sheds tears, and the heart feels sadness, and we do not say except that which pleases the Lord. Indeed, we feel much sadness at your departure, Ibraaheem.” (Reported in Saheeh Al-Jaami’ 2931)
“Truly! Allah is with As-Sabirin (the patient ones).” (2:153)
The opposite of sabr is anger, and resentment. This shows discontentment with the decree of Allah.