Is there a Hadeeth that says that the 'Dhikr of the Heart' (that which the angles cannot hear) earns us Hasanaat 70 times more that the 'Dhikr of the Tongue' (that which the angles can hear?) Please do reply.
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 is His slave and Messenger.
You have to know, dear brother, may Allah enable you to do good, that the mention of Allah is of three kinds: Sometimes it is done with the tongue and heart, and this is the best kind of the mention of Allah, sometimes it is done with the heart only, and this comes at the second degree of excellence, and sometimes it is done with the tongue only and this is the third stage of the mention of Allah.
Therefore, the best mention of Allah it that which is done with the heart and tongue, then at the second degree of excellence is the mention done with the heart only, as this is more fruitful than the mention with the tongue only. Imaam Ibn Al-Qayyim said: 'The mention with the heart only is better than the mention with the tongue only, because the mention done with the heart brings forth knowledge, incites love, increases bashfulness, stimulates the fear of Allah and having Allah-consciousness, and prevents one from being neglectful about acts of obedience and prevents him from indulging in sins and acts of disobedience. Whereas the mention done only with the tongue does not bring forth any of the above results; its fruits are very little and limited.'
Nonetheless, the mention of the tongue has benefits among which is to get it used to the mention of Allah, and the Prophet pointed to this when he said: "Keep your tongue busy with the mention of Allah." [Ahmad] Moreover, it keeps one busy from engaging in falsehood, like backbiting, slandering, and idle talk, as when a person does not engage himself in acts of obedience, his soul [desires] will engage him in falsehood. Moreover, the mention with the tongue only helps to repel the devil and prevents a person from being neglectful.
With regard to your statement about the preference of the mention of the heart over the mention of the tongue by 70 times, then this requires evidence from revelation (i.e. the Quran or the Sunnah ) and we do not know any Prophetic narration which states this.
Finally, it should be noted that the mention with the heart is to feel that Allah is always watching you and knows what you do and what you think about or dream of.
Allah Knows best.