Hermas (14.1.4-14.1.8)

Therefore purify your heart from all the vanities of this world, and from the words which were spoken to you beforehand, and ask from the Lord, and you shall receive all things, and shall not fail to obtain any of your petitions, if you ask from the Lord without doubting.

But if you doubt in your heart, you shall receive none of your petitions. For those who have doubts towards God, these are the double-minded, and they shall not in any wise obtain any of their petitions.

But they who are perfect, in faith ask for all things, trusting in the Lord, and they receive them, because they ask without doubting, and are double-minded in nothing. For every double-minded man, unless he repent, shall with difficulty be saved.

Therefore purify your heart from double-mindedness, but put on faith, because it is mighty, and believe God, that you shall obtain all your requests which you make. And if ever you make any petition from the Lord, and receive it but slowly, do not be double-minded because you have not received the request of your soul speedily, for in every case it is because of some temptation or some transgression, of which you are ignorant, that you receive your request slowly.

Do not therefore cease from making the request of your soul, and you shall receive it. But if you grow weary, and are double-minded in your request, blame yourself and not him who gives to you.