It is not permissible to swear by salaah (prayer) or by honesty
Question

Is it permissible to affirm what you are saying to your brother by saying “by your prayer” or “by your honesty”, or by saying, “you will be in a critical state if you do that?” Such things are widespread among women and children. We hope that you can advise us, may Allaah reward you with good.
Praise be to Allah.It is not permissible to swear by prayer, honesty, critical situations or any other created thing. One should swear only by Allaah. One should not say “By my honesty I will do such and such”, or “by the honesty of So and so”, or “by the life of So and so” or “by my prayer”. Nor should one ask anyone else to do this and tell them, “Say ‘by my honesty/by my prayer/ by my zakaah.’” There is no basis for any of this, because prayer is the action of a person and zakaah is the action of a person, and people’s actions are not something to swear by. One should swear only by Allaah or by His attributes, because the Prophet (peace and blessings of Allaah be upon him) said: “Whoever swears, let him swear by Allaah or keep silent.” (Saheeh, agreed upon). And he (peace and blessings of Allaah be upon him) said: “Whoever swears by anything other than Allaah has committed shirk.” (Narrated by Imaam Ahmad with a saheeh isnaad from ‘Umar (may Allaah be pleased with him)). It was also narrated by al-Tirmidhi and Abu Dawood with a saheeh isnaad from Ibn ‘Umar (may Allaah be pleased with him) from the Prophet (peace and blessings of Allaah be upon him), with the wording, “Whoever swears by anything other than Allaah has committed kufr or shirk.” And the Prophet (peace and blessings of Allaah be upon him) said: “Whoever swears by honesty is not one of us.” So every believing man and woman has to beware of that and not swear by anything except Allaah alone, so he should say “Wallaahi (by Allaah) I did not do such and such”, etc., if that is necessary. What is recommended in sharee’ah is to be cautious about swearing oaths, and not to swear except when essential. Allaah says (interpretation of the meaning):
“And protect your oaths (i.e. do not swear much)”[al-Maa’idah 5:89]
 But if it is necessary, then one may swear by saying, “By Allaah, I did not do such and such”, provided that he is telling the truth, or, “By Allaah, I did not go to So and so” etc. If he is telling the truth, then there is nothing wrong with that, because this is swearing by Allaah when there is a need to do so. But swearing by honesty or by the Prophet (peace and blessings of Allaah be upon him), by the Ka’bah, or by the life of So and so, or by the honour of So and so, or by my prayer, or by my honesty – this is not permissible, as stated above, because of the ahaadeeth quoted.
 But if a person says “Trust me (lit. in my trust)”, this is not an oath; this means that this is something which has been entrusted to me and for which I am responsible. But if a person says “by my trust/ by my prayer/ by my father’s life”, this is not permissible, because he is swearing by something other than Allaah. We ask Allaah to guide us all.