Ka’b ibn Ujrah narrated that the Prophet, sallallahu ‘alayhi wa sallam, said, “There are Mu‘aqqibaat, the one who says them shall not be miserable. Glorify Allaah at the end of every prayer thirty-three times, and praise him thirty-three times, and extol His greatness thirty-four times.” If I do not say this after the prayer but say it at other times, will I still not be miserable?

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger. 

It is recommended to recite this Thikr (expressions of remembrance of Allah) mentioned in the hadeeth (cited in Saheeh Muslim and other books) after the obligatory prayer because this makes the reward greater. Al-Munaawi wrote about the hadeeth in reference, “This hadeeth indicates that it is recommended to recite this Thikr after the obligatory prayers, and the rationale is that at the time of obligatory prayers, the gates of heaven are opened and the deeds are raised to the Lord of the worlds. Therefore, reciting Thikr is most likely accepted and yields greater rewards then.” [Faydh Al-Qadeer]

As for the one who recites this Thikr at other times (not after the obligatory prayers), he shall not be miserable; rather, he earns great rewards, because Tasbeeh (saying 'Subhaan Allah', meaning: Glorified be Allah), Tahmeed (saying 'Al-Hamdulillah' meaning: praise be to Allah), and Takbeer (saying 'Allahu Akbar', meaning: Allah is The Greatest) are among the most beloved speech to Allah. Samurah ibn Jundub, may Allah be pleased with him, reported that the Messenger of Allah, sallallahu ‘alayhi wa sallam, said, “The most beloved speech to Allah are four: Subhaan Allah (Glorified be Allah), Al-Hamdulillah (Praise be to Allah), Laa ilaaha illa Allah (None is worthy of worship but Allah), Allahu Akbar (Allah is The Greatest). There is no harm on you regarding which of them you begin with.” [Muslim and others]

Mirqaat Al-Mafateeh reads, “They were called Mu‘aqqibaat because they are repeated time after time, or because they are recited after the obligatory prayers. The one who recites them shall never be miserable, meaning that he shall not be denied the reward that they yield in any case, even if he recites them in a heedless state. This is the apparent indication of the hadeeth's wording, and Allah Almighty knows best.”

Allah knows best.